SUMMARY OF WORK During the previous project period we developed an algorithm to simulate stochastic local control of cardiac excitation-contraction coupling. For reasons of available computational power, most of these simulations were done using a simplified version of the algorithm which ignores the dynamics of diffusion of calcium ions in the diadic junctional cleft, and therefore also cannot take into account the effects of local calcium buffers and bi-directional flux of calcium ions through channels. During this project period, the algorithm has been re-coded to make use of the MPI (message passing interface) system which makes it possible to distribute the computations in parallel among multiple workstations linked by fast ethernet. This major redevelopment has resulted in a 10-fold increase in computational speed, which has made it practical to use the full dynamic-diffusion simulation. Studies with dynamic diffusion are underway, and have confirmed that the time-dependence of the spread of local calcium through the cleft has an important effect on the macroscopic behavior of excitation-contraction coupling, and that both fixed and diffusible calcium buffers, including endogenous ATP, have major effects. Most importantly, the simulations showed that SR calcium release fluxe in response to brief tail currents through the L-type sarcolemmal calcium channel carry a signature that directly reflects the dynamics of spread of calcium-induced calcium release (CICR) among the ryanodine receptors in the diad ? a signature which we had actually previously observed experimentally without realizing its significance. Simulations predict that this signature can be manipulated with exogenous diffusible buffers, which will make possible a series of experiments that can directly probe the local regenerative CICR which is at the heart of cardiac EC coupling.